(Reuters) - A U.S. Christian group that once promoted "conversion therapy" to encourage gays and lesbians to overcome their sexual preferences has closed its doors and apologized to those who underwent treatment, acknowledging its mission had been hurtful and ignorant.

Exodus International billed itself as the oldest and largest Christian ministry dealing with faith and homosexuality, operating since 1976.

The group's board unanimously voted to cease operations and begin a separate ministry, Exodus International said in a statement on its website on Wednesday.

"We have made a number of mistakes with how we treated people, based on our beliefs," President Alan Chambers told Reuters on Thursday. "I recognize that our beliefs have to change, but I'd never distance myself from the church."

Chambers declined to estimate how many people underwent therapy, saying it was impossible to calculate because it was practiced by some 260 Exodus International-affiliated ministries across North America.

"I am sorry for the pain and hurt that many of you have experienced," Chambers, who said he was part of a "system of ignorance," said in the statement.

"I am sorry some of you spent years working through the shame and guilt when your attractions didn't change. I am sorry we promoted sexual orientation change efforts and reparative theories about sexual orientation that stigmatized parents."

Chambers, who lives in Orlando, Florida, with his wife and two children, said in the statement that for several years he "conveniently omitted" his own "ongoing same-sex attractions."

"I was afraid to share them as readily and easily as I do today," Chambers wrote.

SHIFTING ATTITUDES

Exodus International has closed at a time of shifting attitudes in the United States, with public opinion polls tilting in favor of same-sex marriage.

Twelve states and the District of Columbia have legalized same-sex marriage. The U.S. Supreme Court is expected to rule by the end of June on a challenge to the Defense of Marriage Act, a 1996 law that restricted federal recognition of marriage to heterosexual couples, as well as a challenge to a 2008 California referendum that banned same-sex marriage in that state.

Ross Murray, a spokesman for gay rights group GLAAD, called the closing of Exodus International a step in the right direction and welcomed Chambers' move away from "divisive and demonizing rhetoric."

"But it's going to take a long time for healing to come, especially for the people who have gone through Chambers' program and have suffered because of it," Murray said.

Exodus International's mission statement was "mobilizing the body of Christ to minister grace and truth to a world impacted by homosexuality."

The group appeared to have changed its views incrementally, culminating with the announcement of its closure at the group's 38th annual conference on Wednesday.

In interviews last year with the New York Times and MSNBC, Chambers said churches had shown an overemphasis on sexuality.

A television program scheduled to run on the Oprah Winfrey Network on Thursday will show Chambers meeting with people who said they were harmed by his therapy.

"Mine was not a change of heart, but it had been a change of how we talk about what we have in our hearts," Chambers told Reuters. "So often the religious message is that gay people aren't welcome, and it became something I just couldn't stand by."

California last year became the first U.S. state to ban such therapy for minors. A lawsuit challenged the law, and in December a federal appeals court put the ban on hold. The case is pending before the 9th U.S. Circuit Court of Appeals.

(Additional reporting by Alex Dobuzinskis in Los Angeles and Victoria Cavaliere in New York; Editing by Lisa Von Ahn and Stacey Joyce)